Have you looked to see if the Bitlocker Key was stored as a file on the device? I know this is not recommended when you create the key, but it doesn't mean it is not done. The creation date of the .txt might help. Or you could look to see if there is a link file to a USB drive where the file was stored. Many times people will check a file after it is transferred to an external drive to make sure it will open. I know these are low-tech solutions, but sometimes they are effective.